15 years of experience stator ground fault protection

15 YEARS OF EXPERIENCEWITH

100% GENERATOR STATOR GROUND FAULT PROTECTIONWHAT WORKS, WHAT DOESN’T AND WHY



Chuck MozinaConsultant

Beckwith Electric

BACKGROUND:

Until the Late ’70’s 100% Stator Ground Schemes Were Not Widely Used in the US.

Users Accepted Ground Fault Coverage of Only 90-95% of the Stator Winding.

Conventional 59G Neutral Overvoltage Protection Used.

Stator Ground Fault Protection

100% Schemes Using 3rd Harmonic Neutral Undervoltage Introduced in the Late ’70’s.

Early ’80’s 100% 3rd Harmonic Ratio Scheme was Introduced.

During this Period 100% Subharmonic Injection Schemes Were Being Used in Europe

BACKGROUND:


Conventional 59G Stator Ground Fault Application and Settings.

Undervoltage Neutral 3rd Harmonic scheme for 100% Stator Ground Fault Protection.

3rd Harmonic Ratio Scheme for 100% Stator Ground Fault Protection.

Limitation of These 3rd Harmonic Schemes and Some Methods of Overcoming these Limitations.

Subharmonic Injection Method for 100% Stator Ground Fault Protection.

PAPER ADDRESSES:


CONVENTIONAL (59G) STATOR GROUND FAULT

PROTECTION

CONVENTIONAL (59G) STATOR GROUND FAULT

PROTECTION


Conventional Stator Ground Fault Protection

Fault Position

Volta

ge a

t Neu

tral

(60

Hz)

0% 50% 100%0

0.5pu

1.0pu

N T

90-95%

59G


59G Coordination Issues

F1 – TRANSMISSION SYSTEM GROUND FAULTF2 – VT SECONDARY GROUND FAULT

CH-L

F1

F2

59G


Calculation of Generator Neutral Voltage for a Transmission System Ground Fault

C0

CH-L

VHO

Where:

= High side zero sequence voltage for a transmissionsystem ground fault.

= Generator step-up transformer high-to-low windingcapacitance

CH-L

VHO

C0= Generator system zero sequence capacitance perphase

3RN

3RN = Equivalent primary resistance in generator neutral

VR


VT Connection to Avoid 59G Coordination with VT Fuses

59G


3rd HARMONICSTATOR GROUNDFAULT DETECTION

SCHEMES

3rd HARMONICSTATOR GROUNDFAULT DETECTION

SCHEMES


100% SCHEMES USING 3RD HARMONIC

Undervoltage 3rd Harmonic Scheme

3rd Harmonic Ratio Scheme


Level of 3rd Harmonic Voltage Depends on:

The Construction of the Generator-- The Pitch of the Stator Windings

The MW and MVAR Output of the Generator

The Generator Terminal Capacitance


General 3rd Harmonic Behavior


3rd HARMONIC NEUTRAL

UNDERVOLTAGESCHEME

3rd HARMONIC NEUTRAL

UNDERVOLTAGESCHEME


Third Harmonic Neutral Undervoltage Scheme

59G

59G


Overlapping of 59G and 27TN to Provide 100% Stator Ground Fault Coverage

59G

59G


Field Measurements of the Magnitude of Third Harmonic Neutral Voltage


Advantages / Disadvantages:

Low Cost and Simple.

No New Equipment.

Requires Field Testing to Determine Minimum 3rd

Harmonic.

May Have to be Blocked During Some Operating Conditions.

Will not Work at all if 3rd Harmonic is Low – False Tripping.


3rd HARMONICRATIO SCHEME 3rd HARMONICRATIO SCHEME


3rd Harmonic Ratio Scheme

5 9 G

5 9 D T h ir d H a rm o n icR a t io R e la y

C o n v e n t io n G r o u n dO v e r v o lta g e R e la y

3 V 0


Field Measurements of the Magnitude of Third Harmonic Voltages



More Secure Than Using Only Neutral 3rd Voltage.Requires Measurement of 3rd Harmonic Terminal Voltage.Requires Wye Grounded Primary VT with Broken Delta Secondary.This Generally Requires VT Additions or Modifications and Additional Wiring.Requires Extensive Field Testing to Determine Maximum 3rd Harmonic Ratio.Will Not Work at all if 3rd Harmonic is Low – But No False Tripping.


LIMITATIONS OF

3rd HARMONICSCHEMES

LIMITATIONS OF

3rd HARMONICSCHEMES


Hydro Plant Applications:

False Tripping during Load Rejection

Synchronous Condenser Operation

Performance of Third Harmonic Scheme on Parallel Generators


Parallel Generators on a Common GSU

BUS

G1

G2

59G

27TN

59G

27TN

GSU


Cross Compound and Large Generators Applications

27TN

W inding 1W inding 2

W inding not covered by27TN

G S U


Gas Turbine Third Harmonic Voltage Profile


Third Harmonic Neutral Undervoltage(27TN) Blocking


SUBHARMONIC VOLTAGEINJECTIONSCHEME

SUBHARMONIC VOLTAGEINJECTIONSCHEME


Subharmonic Voltage Injection Scheme

*

C o u p lin gF ilte r

V o lta g eIn je c to r

R e la yM e a s u re m e n ts

I

V

N a tu ra l C a p a c ita n c e

5 9 GN o te s :

S u b h a rm o n ic in je c t io n ty p ic a lly a t 1 5 -2 0 H zC o u p lin g f i l te r lo w p a s s o r n o tc h tu n e d fo r s u b h a rm o n ic f re q u e n c yM e a s u re m e n t in p u ts tu n e d to re s p o n d to s u b h a rm o n ic fre q u e n c y


Coupling Filter and Voltage Injector Stator Ground Fault Protection


Does Not Use 3rd Harmonic Voltage. Independent of Gen. MW and MVAR Loading.Can Detect Stator Ground Fault When Gen. is Off-Line.Detects Stator Ground Faults Over the Entire Winding–No Blind Spots. Requires Field Measurement of Subharmonic Charging Current – Off-Line Measurement.Requires Additional Equipment in Grounding Transformer Secondary – Added Cost.Cost have Reduced With Digital Technology.


CONCLUSIONSIt’s Important to Upgrade to 100% Stator Ground Fault Protection.If 3rd Harmonic Schemes are to Being Considered-- It is Important to First Determine 3rd Harmonic “Signature”of the Generator.Recognize that 3rd Harmonic Schemes Have Limitations. Subharmonic Injection Provides an Viable Alternative to 3rd Harmonic Schemes.Injection Scheme Cost have Reduced with Digital Technology – Good Choice for Important Generators.Injection Scheme can Detect Stator Grounds Over the Entire Stator Winding and in an Off-line Generator.


THE END

Questions ?

THE END

Questions ?






©2008 Beckwith Electric Co., Inc.

15 years of experience stator ground fault protection

Engineering